This invention relates to the origination of a telephone call, including the dialing of a distant party, from a mobile station and, wherein, the dialed number is transmitted to the base station via the reverse traffic channel to prevent identification of the dialed digits by other stations.
One form of communication which is of interest and of great utility in the operation of cellular telephone systems is communication via CDMA (code division multiple access). Such a form of communication is currently in use in numerous cellular telephone systems. In the CDMA form of communication, signals transmitted and received by a mobile telephone comprise digitally encoded messages modulated onto carriers using a spread spectrum. Reverse traffic and the access channels are direct sequence spread by use of a long code which is a pseudo-random binary sequence. Each chip of the long code is generated by a long code generator, and the particular long code sequence is determined by a long code mask input to the long code generator.
The operation of CDMA cellular telephone communication in accordance with IS-95 is well known, this being the mobile station-base station compatibility standard for dual-mode wide band spread spectrum cellular system of IS-95A. The long code mask for the access channel is generally known by any mobile station. Due to the long code mask of the access channel being known, interception and decoding of messages transmitted on the access channel is possible and relatively simple. The reverse traffic is more secure because the long code mask is not generally known. The identification of all telephones is accomplished by assigning different long code masks to the respective telephones.
Four channels are used for two-way communication between mobile station and base station, these channels being a paging channel, an access channel, a forward traffic channel, and a reverse traffic channel. The paging channel and the forward traffic channel are on a first carrier frequency or spectrum, and serve for communication from the base station to a mobile station. The access channel and the reverse traffic channel are on a second carrier frequency or spectrum, and serve for communication from the mobile station to the base station. The paging channel is distinguishable from the forward traffic channel by virtue of the Walsh coding; and the access channel is distinguishable from the reverse traffic channel by virtue of the Walsh coding. The first and the second carrier frequencies, or spectra, are shared by all mobile telephones communicating with a common base station. The spectral separation of the two carrier frequencies is sufficient to allow independent operation of the telephone transmission and receiving circuits.
To initiate a telephonic communication, a mobile station employs the access channel to contact the base station, and to supply to the base station the dialing number of the called party and the identification of the mobile station. Subsequent communication between the calling and the called parties via the base station is accomplished via the traffic channels. In the use of the traffic channels, the long code mask is made private and makes traffic data unintelligible to an intruder. Decryption is available only to the base station since only the base station knows the private long code mask. Such encryption is not employed in the use of the access channel. As a result, the identifications of the called and the calling parties can be obtained by an intruder having suitable receiving equipment for listening to the telephone traffic.
Thus a problem exists in that, generally, people prefer to keep their telephone communications private. However, at the present time, an intruder can identify the called parties.